Organic electronic devices, such as organic thin-film transistors (OTFTs) and other organic semiconductors (OSCs) have attracted a great deal of attention because of their potential applications in low-cost, large-area and flexible electronics. Organic semiconductors are commonly referred to as either p-channel (hole-transporting) or n-channel (electron-transporting) depending on which type of charge carrier is dominant for the charge transport.
While p-channel organic semiconductors have been readily implemented for a variety of applications, n-channel organic semiconductors have been challenging to manufacture and implement. Generally, the energetically high-lying lowest unoccupied molecular orbital (LUMO) levels in most organic semiconductors hinder the efficient injection of electrons. In n-channel OTFTs, electrons can be transferred from the high-lying highest occupied molecular orbitals (HOMOs) of dopants to the LUMOs of organic semiconductors by n-type doping. However, such dopants are susceptible to oxidation in air. Electron charge carriers are vulnerable to trapping, either by traps at the interface of dielectric-semiconductor materials, which may involve hydroxyl groups or ambient oxidants, such as O2 and H2O. Such charge-trapping can decrease the density of mobile electron charge carriers, degrade the mobility, and increase the threshold voltage for n-channel OTFTs.
While various n-type dopants have been used to dope organic semiconductors, they have been challenging to implement. For example, alkali metals are prone to diffuse through organic layers due to their relatively small atomic radii, leading to device instability. In addition, alkali metals are difficult to process. Other dopants having both extremely high-lying HOMO levels and exhibiting air stability do not provide donors that are strong enough to obtain sufficient n-channel conductivity. Cationic dyes have been used as stable precursors for strong molecular donors, but have been relatively limited due to the lack of available compounds having a strong n-type doping effect.
These and other issues remain as a challenge to a variety of methods, devices and systems that use or benefit from organic semiconductors, such as organic thin-film transistors, organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs).